1. Field of the Invention
The present invention relates to a receiver device, and particularly to a receiver device employing a single conversion system.
2. Description of the Background Art
Fifty years have passed since analog television broadcasting started, and twenty to thirty millions of homes have television sets at present. In Japan, digital terrestrial broadcasting finally started in the year 2003, and analog broadcasting will end in the year 2001. Cable television and BS (Broadcasting Satellite) are being switched to digital systems.
A tuner for receiving television signals have receiver circuits dedicated to different bands, respectively, and for example, a tuner for the United States has receiver circuits, which are dedicated to a UHF (Ultra High Frequency) band of 470-860 MHz, a VHF (Very High Frequency) high band of 170-470 MHz and a VHF low band for 54 to 170 MHz. However, the manner of dividing the bands depends on destinations of products, and is not particularly defined. Conventional tuners for receiving television signals generally employ a single conversion system (superheterodyne system).
FIG. 10 is a schematic block diagram schematically showing a block structure of a conventional tuner 100 of a single conversion type.
Referring to FIG. 10, conventional tuner 100 of the single conversion type includes an IF (Intermediate Frequency) filter 101, an input circuit 102, a UHF input tuning circuit 103, an AGC (Automatic Gain Control) resistance 104, a UHF high-frequency amplifier 105, a UHF output tuning circuit 106, a UHF mixer circuit 107, a UHF local oscillator circuit 108, a VHF mixer circuit 118, a local oscillation signal select switch 119, VHF local oscillator circuits 120 and 121, an IF amplifier circuit 122, a VHF input tuning portion 200 and a VHF output tuning portion 300.
IF filter 101 is a high-pass filter, which allows passage of only a high frequency portion, e.g., of 54 MHz or higher of a television signal received from an input terminal (antenna terminal). Input circuit 102 extracts a UHF signal from the television signal passed through IF filter 101. UHF input tuning circuit 103 performs input tuning of the UHF signal extracted by input circuit 102. AGC resistance 104 is connected between an AGC0 terminal and UHF high-frequency amplifier 105. The AGC0 terminal receives a gain control signal from a video signal demodulation circuit (not shown).
UHF high-frequency amplifier 105 receives the gain control voltage applied from AGC0 terminal via AGC resistance 104, and amplifies the UHF signal, which underwent the input tuning by UHF input tuning circuit 103. UHF output tuning circuit 106 performs output tuning of UHF signal amplified by UHF high-frequency amplifier 105. UHF mixer circuit 107 receives a local oscillation signal from UHF local oscillator circuit 108, and provides an IF signal by performing frequency conversion of the UHF signal, which underwent the output tuning by UHF output tuning circuit 106.
VHF input tuning portion 200 receives the VHF signal in the television signal passed through IF filter 101. VHF input tuning portion 200 includes a high/low band select switch 109, a VHF high-band input tuning circuit 110, a VHF low-band input tuning circuit 111, an AGC resistance 112 and a VHF high-frequency amplifier 113.
High/low band select switch 109 selects the VHF signal as a VHF high-band signal or a VHF low-band signal. VHF high-band input tuning circuit 110 performs input tuning of the VHF high-band signal. VHF low-band input tuning circuit 111 performs input tuning of the VHF low-band signal. AGC resistance 112 is connected between the AGC0 terminal and VHF high-frequency amplifier 113. VHF high-frequency amplifier 113 receives the gain control voltage applied from the AGC0 terminal via AGC resistance 112, and amplifies the VHF signals, which underwent the input tuning by VHF high-band input tuning circuit 110 and VHF low-band input tuning circuit 111.
VHF high-frequency amplifier 113 provides the VHF signal amplified thereby to VHF output tuning portion 300. VHF output tuning portion 300 includes high/low band select switches 114 and 117, a VHF high-band output tuning circuit 115 and a VHF low-band output tuning circuit 116.
High/low band select switch 114 selects the VHF signal amplified by VHF high-frequency amplifier 113 to provide it as a VHF high-band signal or a VHF low-band signal. VHF high-band output tuning circuit 115 performs the output tuning of the VHF high-band signal. VHF low-band output tuning circuit 116 performs the output tuning of the VHF low-band signal. High/low band select switch 117 performs the switching between the VHF high-band signal output-tuned by VHF high-band output tuning circuit 115 and the VHF low-band signal output-tuned by VHF low-band output tuning circuit 116.
VHF mixer circuit 118 receives a local oscillation signal sent from VHF local oscillator circuit 120 or a local oscillation signal sent from VHF local oscillator circuit 121, and provides the IF signal by performing the frequency conversion of the VHF signal, which underwent the output tuning by VHF output tuning portion 300. Local oscillation signal select switch 119 selects the local oscillation signal sent from VHF local oscillator circuit 120 and the local oscillation signal sent from VHF local oscillator circuit 121 depending on whether the VHF signal is a high-band signal or a low-band signal. IF amplifier circuit 122 amplifies the IF signal, which underwent the frequency conversion by UHF mixer circuit 107 or VHF mixer circuit 118, and provides it to an output terminal.
In conventional tuner 100 of the single conversion type, a PLL (Phase Locked Loop) circuit (not shown) performs a practical tuning or channel selecting operation. A portion of a specific circuit structure of tuner 100 will now be described.
FIG. 11 is a circuit diagram fragmentarily showing the specific circuit structure of conventional tuner 100 of the single conversion type.
Referring to FIG. 11, conventional tuner 100 of the single conversion type includes IF filter 101, input circuit 102 and VHF input tuning portion 200. IF filter 101 is connected to input circuit 102 and VHF input tuning portion 200 via a DC blocking capacitor C101.
Input circuit 102 includes a UHF/VHF switching diode D101, and bias resistances R101 and R102. UHF/VHF switching diode D101 is connected between nodes N101 and N102, and is arranged in the forward direction with respect to node N101. Bias resistance R101 is connected between node N101 and a ground node. Bias resistance R102 is connected between node N102 and a BuO terminal. The BuO terminal is a power supply terminal for the UHF signal. Input circuit 102 (node N102) is connected to UHF input tuning circuit 103 via a DC blocking capacitor C102 (see FIG. 10).
VHF input tuning portion 200 includes RF(Radio Frequency) choke coils L201 and L206, bypass capacitors C201, C203, C205 and C207-C209, a VHF low-band tuning coil L202, a VHF low-band matching coil L203, a damping resistance R201, DC blocking capacitors C202, C204 and C206, a high/low band switching diode D201, a VHF high-band matching coil L204, a VHF high-band tuning coil L205, a tuning variable-capacitance diode D202 (i.e., a variable-capacitance diode for tuning), a coupling variable-capacitance diode D203 (i.e., a variable-capacitance diode for coupling), bias resistances R202 and E203, AGC resistance 112 and VHF high-frequency amplifier 113. VHF high-frequency amplifier 113 is a MOS type Field Effect Transistor (MOS FET).
RF choke coil L201 is connected between node N101 and a BL0 terminal. The BL0 terminal is a power supply terminal for the VHF low-band signal. Bypass capacitor C201 is connected between the BL0 terminal and the ground node. VHF low-band tuning coil L202 is connected between nodes N101 and N105. VHF low-band matching coil L203 is connected between nodes N101 and N103. Damping resistance R20 is connected between nodes N103 and N106. DC blocking capacitor C202 is connected between nodes N103 and N104.
High/low band switching diode D201 is connected between nodes N104 and N106, and is arranged in the forward direction with respect to node N106. VHF high-band matching coil L204 is connected between node N104 and a BH0 terminal. The BH0 terminal is a power supply terminal for the VHF high-band signal. Bypass capacitor C203 is connected between the BH0 terminal and the ground node. DC blocking capacitor C204 and VHF high-band tuning coil L205 are connected in series between nodes N105 and N107.
Tuning variable-capacitance diode D202 is connected between node N107 and the ground node, and is arranged in the forward direction with respect to node N107. Bias resistance R202 is connected between node N107 and a Vt0 terminal. The Vt0 terminal is a tuning terminal for VHF input tuning portion 200. Bypass capacitor C205 is connected between the Vt0 terminal and the ground node. Coupling variable-capacitance diode D203 is connected between nodes N107 and N108, and is arranged in the forward direction with respect to node N107. Bias resistance R203 is connected between node N108 and the ground node.
AGC resistance 112 is connected between a node N109 and the AGC0 terminal. The AGC0 terminal receives the gain control signal from the video signal demodulating circuit (not shown). Bypass capacitor C207 is connected between node N109 and the ground node. Bypass capacitor C208 is connected between the AGC0 terminal and the ground node. VHF high-frequency amplifier (MOS FET) 113 has a source connected to node N109, a drain connected to node N108 via DC blocking capacitor C206 and a back gate connected to the ground node.
RF choke coil L206 is connected between a gate of MOS FET 113 and a +B0 terminal. The +B0 terminal is a power supply terminal for VHF input tuning portion 200. Bypass capacitor C209 is connected between the +B0 terminal and the ground node. VHF input tuning portion 200 (gate of MOS FET 113) is connected to VHF output tuning portion 300 (see FIG. 10).
A conventional television tuner input circuit disclosed in Japanese Utility Model Laying-Open No. 03-128339 has disclosed a feature, in which a p-type high-pass filter formed of a coil and a capacitor is connected in series to a downstream end of a resonant trap formed of a coil and a capacitor, and an input tuning circuit is connected in series to a downstream end of the above high-pass filter via a coupling capacitor.
According to a conventional electronic-tuning tuner disclosed in Japanese Patent Laying-Open No. 04-369918, a voltage supply, which operates an IF amplifier circuit, applies a bias voltage to a gate of a dual gate MOS FET arranged for VHF and UHF amplifier circuits. This electronic-tuning tuner utilizes the fact that the cut-off characteristics during the off state of the dual gate MOS FET are improved when a bias voltage is applied, and thereby suppresses entry of harmonic components of the UHF channel signal into a mixer circuit during the VHF receiving operation as well as entry of harmonic components of the VHF channel signal in the mixer circuit during the UHF receiving operation.
In conventional tuner 100 of the single conversion type shown in FIG. 11, since a circuit corresponding to input circuit 102 on the UHF band side is not present on the VHF band side, an interference signal due to the UHF signal affects the VHF signal sent to VHF input tuning portion 200. This causes a problem in characteristics of selectivity of the received signal in VHF input tuning portion 200 when the VHF low-band signal is received.
Also, conventional tuner 100 of the single conversion type shown in FIG. 11 cannot achieve a sufficiently high image rejection ratio. This causes a problem in characteristics of selectivity of the received signal in VHF input tuning portion 200 when the VHF low-band signal or VHF high-band signal is received.
Further, according to conventional tuner 100 of the single conversion type shown in FIG. 11, tuning diode D202 and coupling variable-capacitance diode D203 are connected equivalently in parallel. This results in a problem that a large tracking deviation occurs when the VHF high-band signal is received. The tracking deviation is a deviation between a local oscillation frequency and a RF (Radio-Frequency) amplifier circuit.
Further, conventional tuner 100 of the single conversion type shown in FIG. 11 suffers from a problem that an interference ratio of about −40 dBc occurs when receiving a US6 channel (video frequency of 83.25 MHz and audio frequency of 87.75 MHz).
Further, conventional tuner 100 of the single conversion type shown in FIG. 11 suffers from a problem that it has a low resistance to interference at a frequency lower than the IF frequency of 5-40 MHz, and particularly at a frequency of 27 MHz in a CB (Citizen Band).
Further, conventional tuner 100 of the single conversion type shown in FIG. 11 suffers from a problem that sufficient removal characteristics cannot be achieved with respect to the UHF signal of a large amplitude in the operation of receiving the VHF signal.
The conventional television tuner input circuit disclosed in Japanese Utility Model Laying-Open No. 03-128339 and the conventional electronic-tuning tuner disclosed in Japanese Patent Laying-Open No. 04-369918 can overcome some of the foregoing problems, but cannot overcome all the foregoing problems. Further, manners of overcoming the problems are not restricted to those disclosed in these publications.